Steel product for induction hardening, induction-hardened member using the same, and methods producing them

ABSTRACT

The present invention provides a steel product that consists of, by mass %, C: 0.35-0.7%, Si: 0.30-1.1%, Mn: 0.2-2.0%, Al: 0.005-0.25%, Ti: 0.005-0.1%, Mo: 0.05-0.6%, B: 0.0003-0.006%, S: 0.06% or less, P: 0.02% or less, Cr: 0.2% or less, and the balance Fe and inevitable impurities, and has a structure of bainite and/or martensite, the total volume fraction of bainite and martensite being 10% or more, and an induction hardened member that is made of the steel product having a hardened surface layer formed by induction hardening and has a prior austenite grain size of 12 μm or less through the layer thickness. The member has high fatigue strength and therefore is suitable for an automobile drive shaft, an automobile constant velocity joint or the like.

TECHNICAL FIELD

The present invention relates to a steel product suitable for anautomobile drive shaft, an automobile constant velocity joint or thelike that is to be induction hardened to have a hardened layer on thesurface, and to an induction hardened member made of the steel product.The present invention also relates to methods for manufacturing thesteel product and the induction hardened member.

BACKGROUND ART

In general, a machine structural member, such as an automobile driveshaft or an automobile constant velocity joint, is provided with highfatigue strength, such as torsional fatigue strength, bending fatiguestrength, and roller pitting fatigue strength, which are all importantcharacteristics for the machine structural member, by working a hotrolled steel bar into a member with a predetermined shape through hotforging, cutting, cold forging and the like, followed by inductionhardening and tempering.

In recent years, further improvement in the fatigue strength of such amachine structural member has been required as the demand for weightreduction of automobile members increases in view of environmentalissues.

Various methods have been proposed to improve the fatigue strength. Forexample, increasing the depth of induction hardening may be expected toimprove the torsional fatigue strength. However, the torsional fatiguestrength levels off at a certain depth, no further improvement beingrealized.

Increasing grain boundary strength is also effective in improving thetorsional fatigue strength. For example, Japanese Unexamined PatentApplication Publication No. 2000-154819 discloses a method fordecreasing the austenite grain size by dispersing a large amount of fineTiC during induction heating.

However, the dispersion of a large amount of fine TiC during inductionheating requires that the steel should be heated to at least 1100° C.for solution treatment of TiC in advance. This results in lowproductivity. Furthermore, only decreasing the austenite grain size bythe dispersion of a large amount of fine TiC is not sufficient tosatisfy the recent demand for torsional fatigue strength.

Japanese Unexamined Patent Application Publication No. 8-53714 disclosesa machine structural component with improved torsional fatigue strength,in which a value A that is calculated from CD/R, γf, Hf, and Hc isadjusted within a predetermined range depending on the C content,wherein CD/R is the ratio of the thickness (case depth) CD of a hardenedlayer formed by induction hardening of the machine structural componenthaving a circular cross section to the radius R of the circular crosssection, the CD/R being limited to 0.3-0.7; γf is prior austenite grainsize through the depth up to 1 mm in the induction hardened layer; Hf isaverage Vickers hardness of an as-quenched component in the CD/R rangeup to 0.1; and Hc is average Vickers hardness at an axial center afterthe induction hardening.

However, since the prior austenite grain size through the depth of thehardened layer is not taken into consideration in this machinestructural component, the recent demand for torsional fatigue strengthis not satisfied, either.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a steel product forinduction hardening which allows higher fatigue strength than everbefore after induction hardening, and an induction hardened member withhigh fatigue strength prepared from the steel product. It is anotherobject of the present invention to provide a method for manufacturingthe steel product and the induction hardened member.

The object can be achieved by providing a steel product for inductionhardening that consists of

C: 0.35-0.7%,

Si: 0.30-1.1%,

Mn: 0.2-2.0%,

Al: 0.005-0.25%,

Ti: 0.005-0.1%,

Mo: 0.05-0.6%,

B: 0.0003-0.006%,

S: 0.06% or less,

P: 0.02% or less,

Cr: 0.2% or less, by mass, and

a balance of Fe and inevitable impurities, and has a structure ofbainite and/or martensite, the total volume fraction of bainite andmartensite being 10% or more, and

an induction hardened member that is made of the steel product whereinthe prior austenite grain size is 12 μm or less through the thickness ofa hardened surface layer formed by induction hardening.

This steel product for induction hardening may be manufactured by amethod comprising the steps of: hot working a steel consisting of thecomponents described above; and cooling the hot worked steel at acooling rate of at least 0.2° C./s.

The induction hardened member may be manufactured by a method comprisingthe step of: subjecting the steel product to induction hardening atleast once, wherein the final induction hardening is performed at aheating temperature of 800-1000° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a relationship between volume fraction of bainite ormartensite and machinability and increase in strength.

FIG. 2 shows a relationship between heating temperature of inductionhardening and prior austenite grain size of hardened layer in Mo-bearingsteel and Mo-free steel.

EMBODIMENTS OF THE INVENTION

The present inventors made extensive research for efficiently improvingthe fatigue strength of a steel product by induction hardening,particularly the torsional fatigue strength as a typical example of thefatigue strength, and obtained the following findings:

(1) Controlling the chemical composition of a steel product within aproper range and adjusting the prior austenite grain size through thethickness of a hardened layer formed by induction hardening to 12 μm orless increase the torsional fatigue strength remarkably. In particular,controlling the Si content and the Mo content within proper rangesincreases the number of nucleation sites of austenite during inductionhardening, inhibits the grain growth of austenite, and effectivelydecreases the grain size of the hardened layer, thus increasing thetorsional fatigue strength. The addition of 0.30 mass % or more Si iseffective in decreasing the prior austenite grain size through thethickness of the hardened layer to 12 μm or less.

(2) Since carbides disperse more finely in bainite or martensite than infetrite+pearlite, when the total volume fraction of bainite andmartensite in a steel product is at least 10% before inductionhardening, the area of ferrite/carbide interface, which is a nucleationsite of austenite during induction heating, increases and thereby theresulting austenite becomes fine. Consequently, this decreases the grainsize of the hardened layer, increases the grain boundary strength, andthus increases the torsional fatigue strength.

(3) The prior austenite grain size can consistently be decreased to 12μm or less through the thickness of the hardened layer by using a steelproduct having a controlled chemical composition and a controlledstructure as described above, and heating the steel product at 800-1000°C., preferably at 800-950° C. for 5 seconds or less during inductionhardening. In particular, the addition of Mo efficiently decreases thegrain size of the hardened layer in this heating temperature range. Inaddition, repetitive induction hardening provides a hardened layerhaving finer grains than single induction hardening.

The present invention is based on these findings and will be describedin detail below.

1. Steel Product for Induction Hardening

1-1. Composition

C: C has the largest effect on the induction hardenability. C increasesthe hardness and the thickness of a hardened layer, and thereby improvesthe torsional fatigue strength. However, when the C content is 0.35 mass% or less, the hardened layer has to be increased in thicknessdramatically to ensure a required torsional fatigue strength. Thiscauses frequent occurrence of quenching cracks and makes the formationof bainite difficult. On the other hand, when the C content is greaterthan 0.7 mass %, the grain boundary strength decreases and therefore thetorsional fatigue strength decreases. This also results in poormachinability, poor cold forgeability, and poor resistance to quenchingcrack. Accordingly, the C content is limited to 0.35-0.7 mass %,preferably to 0.4-0.6 mass %.

Si: Si increases the number of nucleation sites of austenite duringinduction heating, inhibits the grain growth of austenite, and therebydecreases the grain size of the hardened layer. Furthermore, Si inhibitsthe formation of carbides and therefore prevents a reduction in thegrain boundary strength. Si is also suitable for the formation ofbainite. Thus, Si is very effective in increasing the torsional fatiguestrength. However, when the Si content is less than 0.30 mass %, theprior austenite grain size through the thickness of the hardened layercannot be decreased to 12 μm or less under any condition formanufacturing a steel product and any induction hardening condition. Onthe other hand, the Si content greater than 1.1 mass % causes too muchsolid solution hardening of ferrite, resulting in poor machinability andpoor cold forgeability. Accordingly, the Si content is limited to0.30-1.1 mass %, preferably to 0.40-1.0 mass %.

Mn: Mn improves the induction hardenability and therefore is essentialfor the formation of a hardened layer having a certain thickness.However, less than 0.2 mass % of Mn is insufficient for the effect. Onthe other hand, when the Mn content is greater than 2.0 mass %, retainedaustenite after induction hardening increases and thus the surfacehardness decreases. This results in lower torsional fatigue strength.Accordingly, the Mn content is limited to 0.2 mass %, preferably 0.3mass %, to 2.0 mass %. Furthermore, since a high Mn content may causehardening of the as-rolled steel and therefore result in poormachinability, the Mn content is preferably 1.2 mass % or less, and morepreferably 1.0 mass % or less.

Al: Al is effective in deoxidizing steel. Al is also effective ininhibiting the grain growth of austenite during induction heating andthus decreasing the grain size of a hardened layer. However, less than0.005 mass % of Al is insufficient for the effect. On the other hand,the effect levels off at an Al content exceeding 0.25 mass %, increasingthe cost. Accordingly, the Al content is limited to 0.005-0.25 mass %,preferably to 0.05-0.10 mass %.

Ti: Ti combines with inevitable impurity N in steel and thereby preventsB from forming BN and losing its effect on the induction hardenability,as described below. This requires at least 0.005 mass % of Ti. However,when the Ti content is greater than 0.1 mass %, TiN is produced inexcess and becomes origin of fatigue fracture, which remarkablydecreases the torsional fatigue strength. Accordingly, the Ti content islimited to 0.005-0.1 mass %, preferably to 0.01-0.07 mass %.Furthermore, to ensure the precipitation of N as TiN, and utilize thehardening effect of B effectively to produce a structure of bainite andmartensite, the Ti content and the N content are preferably controlledto satisfy the equation of Ti (mass %)/N (mass %)≧3.42.

Mo: Mo enhances the formation of bainite after hot working, decreasesthe austenite grain size during induction heating, and decreases thegrain size of a hardened layer. Furthermore, Mo inhibits the graingrowth of austenite during induction heating and thus decreases thegrain size of the hardened layer. In particular, when the inductionheating is conducted at 800-1000° C., preferably at 800-950° C., thegrain growth of austenite can be inhibited significantly. In addition,Mo is effective in improving the induction hardenability and istherefore used to adjust the hardenability. Furthermore, Mo inhibits theformation of carbides at a grain boundary and thus prevents a decreasein the grain boundary strength.

As such, Mo plays a very important role in the present invention.However, when the Mo content is less than 0.05 mass %, the prioraustenite grain size through the thickness of the hardened layer cannotbe decreased to 12 μm or less under any condition for manufacturing asteel product and any induction hardening condition. On the other hand,when the Mo content is greater than 0.6 mass %, the hardness of a steelproduct increases remarkably during rolling. This results in poorworkability. Accordingly, the Mo content is limited to 0.05-0.6 mass %,preferably to 0.1-0.6 mass %, and more preferably to 0.3-0.4 mass %.

B: B enhances the formation of bainite or martensite. A small quantityof B improves the induction hardenability, increases the thickness of ahardened layer, and thus increases the torsional fatigue strength.Furthermore, B segregates preferentially at a grain boundary anddecreases the concentration of P segregated at the grain boundary, thusincreasing the grain boundary strength and the torsional fatiguestrength. However, less than 0.0003 mass % of B is insufficient for theeffects. On the other hand, the effects level off at a B content of0.006 mass %, increasing the cost. Accordingly, the B content is limitedto 0.0003-0.006 mass %, preferably to 0.0005-0.004 mass %, and morepreferably to 0.0015-0.003 mass %.

S: S precipitates as MnS, which improves the machinability of steel.When the S content is greater than 0.06 mass %, S segregates at a grainboundary and thus decreases the grain boundary strength. Accordingly,the S content is limited to 0.06 mass % or less, and preferably to 0.04mass % or less.

P: P segregates at an austenite grain boundary, and thus decreases thegrain boundary strength and the torsional fatigue strength. Furthermore,P increases quenching cracks. Accordingly, the P content is limited to0.020 mass % or less and is preferably as low as possible.

Cr: Cr stabilizes carbides and thus enhances the formation of carbidesat a grain boundary, decreases the grain boundary strength, and thusdecreases the torsional fatigue strength. Accordingly, the Cr content islimited to 0.2 mass % or less, preferably to 0.05 mass % or less, and ismore preferably as low as possible.

In addition to the composition described above, further comprising atleast one selected from the group consisting of

Cu: 1.0 mass % or less,

Ni: 3.5 mass % or less,

Co: 1.0 mass % or less,

Nb: 0.1 mass % or less, and

V: 0.5 mass % or less,

is effective in increasing the torsional fatigue strength for thefollowing reasons.

Cu: Cu is effective in improving the induction hardenability.Furthermore, Cu dissolves in ferrite and increases the torsional fatiguestrength by solid-solution strengthening. In addition, Cu inhibits theformation of carbides and prevents a decrease in the grain boundarystrength, thus increasing the torsional fatigue strength. However, theCu content greater than 1.0 mass % causes cracking during hot working.Accordingly, the Cu content is limited to 1.0 mass % or less, andpreferably to 0.5 mass % or less.

Ni: Ni improves the induction hardenability and is therefore used tocontrol the hardenability. Furthermore, Ni inhibits the formation ofcarbides and prevents a decrease in the grain boundary strength, thusimproving the torsional fatigue strength. However, since Ni is a veryexpensive element, the Ni content greater than 3.5 mass % considerablyincreases the cost of a steel product. Accordingly, the Ni content islimited to 3.5 mass % or less. In addition, since less than 0.05 mass %of Ni only achieves a minor improvement of hardenability or a smalleffect of preventing a decrease in the grain boundary strength, the Nicontent is preferably at least 0.05 mass %, and more preferably 0.1-1.0mass %.

Co: Co inhibits the formation of carbides and prevents a decrease in thegrain boundary strength, thus improving the torsional fatigue strength.However, Co is a very expensive element and the Co content greater than1.0 mass % considerably increases the cost of a steel product.Accordingly, the Co content is limited to 1.0 mass % or less. Inaddition, since less than 0.01 mass % of Co has a minor effect ofpreventing a decrease in the grain boundary strength, the Co content ispreferably at least 0.01 mass %, and more preferably 0.02-0.5 mass %.

Nb: Nb not only improves the induction hardenability, but also combineswith C or N to improve the strength by precipitation hardening.Furthermore, Nb increases the resistance to temper softening. Theseeffects increase the torsional fatigue strength, although they level-offat a Nb content of 0.1 mass %. Accordingly, the Nb content is limited to0.1 mass % or less. In addition, since less than 0.005 mass % of Nbachieves a weak precipitation hardening and only a minor improvement inthe resistance to temper softening, the Nb content is preferably atleast 0.005 mass %, and more preferably 0.01-0.05 mass %.

V: V combines with C or N to improve the strength by precipitationhardening. Furthermore, V increases the resistance to temper softening.These effects increase the torsional fatigue strength, although theylevel off at a V content of 0.5 mass %. Accordingly, the V content islimited to 0.5 mass % or less. In addition, since less than 0.01 mass %of V achieves only a small increase in the torsional fatigue strength,the V content is preferably at least 0.01 mass %, and more preferably0.03-0.3 mass %.

1-2. Structure

To improve the torsional fatigue strength by induction hardening, inaddition to the composition of a steel product described in section 1-1,the steel product should have a structure of bainite and/or martensitebefore induction hardening, the total volume fraction (percent byvolume) of bainite and martensite being at least 10%, preferably atleast 20% for the following reason. Since carbides disperse more finelyin a structure of bainite or martensite than in a structure offerrite+pearlite, the area of ferrite/carbide interface, which is anucleation site of austenite during induction heating, increases andthereby the resulting austenite becomes smaller. Consequently, thisdecreases the grain size of a hardened layer, increases the grainboundary strength, and thus increases the torsional fatigue strength.

When the total volume fraction of bainite and martensite exceeds 90%,not only may the decrease in the prior austenite grain size in thehardened layer level off, but also the machinability may deterioratesignificantly. Hence, the total volume fraction of bainite andmartensite is preferably 90% or less.

FIG. 1 shows a relationship between volume fraction of bainite ormartensite and machinability and increase in strength.

In view of an increase in strength, that is, a decrease in grain size ofa hardened layer, bainite has a comparable effect as martensite; atleast 10% of volume fraction of bainite or martensite will be sufficientto achieve a significant increase in the torsional fatigue strength. Onthe other hand, in view of machinability or hardness, bainite issuperior to martensite: 25-85% by volume, preferably 30-70% by volume ofbainite provides both higher strength and excellent machinability.

From the industrial point of view, bainite also has the advantage overmartensite in that bainite is formed with a lesser amount of alloyingelements at a lower cooling rate.

The remaining structure other than bainite and martensite is not limitedand may be ferrite and/or pearlite.

2. Induction Hardened Member

To manufacture an induction hardened member with high torsional fatiguestrength by induction hardening the steel product that has such acomposition and structure, the prior austenite grain size of a hardenedlayer formed on the surface of the member should be 12 μm or less,preferably 10 μm or less, and more preferably 5 μm or less through thethickness of the hardened layer. When the prior austenite grain size ofthe hardened layer exceeds 12 μm, the grain boundary strength isinsufficient, and the improvement in torsional fatigue strength is notexpected.

The prior austenite grain size through the thickness of the hardenedlayer was determined as described below.

After induction hardening, the top surface layer of the member has 100%by area of martensite. The martensite continues to exist at 100% by areato a certain depth from the top and then decreases rapidly. In themember after induction hardening, a layer between the top surface and aregion having 98% by area of martensite is herein referred to as ahardened layer, and the average depth from the top is regarded as thedepth of the hardened layer.

Average prior austenite grain sizes at one-fifth, a half, andfour-fifths of the depth of the hardened layer were measured. When allthe average prior austenite grain sizes were 12 μm or less, the prioraustenite grain size through the thickness of the hardened layer wasconsidered to be 12 μm or less.

The cross-section of the hardened layer was exposed to an etchant thatwas prepared by adding 11 g of sodium dodecylbenzenesulfonate, 1 g offerrous chloride, 1.5 g of oxalic acid and 50 g of picric acid to 500 gof water. The average prior austenite grain size was determined byobserving five fields of view of the etched cross-section for eachposition with an optical microscope at a magnification from 400× (areaof one field of view: 0.25 mm×0.225 mm) to 1000× (area of one field ofview: 0.10 mm×0.09 mm) and using an image analyzer.

If the fatigue strength depends only on the outermost structure, likerolling contact test, even a hardened layer having a thickness of about1 mm has some effect. However, for torsional fatigue strength, thehardened layer preferably has a thickness of at least 2 mm, morepreferably at least 2.5 mm, and most preferably at least 3 mm.

3. Method for Manufacturing a Steel Product for Induction Hardening

Steel having the above mentioned composition according to the presentinvention is processed by hot working, such as rolling or forging, intoa predetermined shape, and then cooled at an average cooling rate of atleast 0.2° C./s to yield a steel product. This steel product has astructure of bainite and/or martensite and is suitable for inductionhardening, the total volume fraction of bainite and martensite being atleast 10%.

The hot working at a temperature of 900° C. or less does not yield arequired structure of bainite and/or martensite. On the other hand, thehot working at a temperature over 1150° C. results in higher heatingcost. Accordingly, the hot working is preferably performed at from morethan 900° C. to 1150° C. The cooling rate after the hot working ispreferably 0.3-30° C./s.

4. Method for Manufacturing an Induction Hardened Member

The steel product for induction hardening having the composition and thestructure as described above according to the present invention is coldrolled, cold forged, or cut, if necessary, and is subjected to inductionhardening at least once. The final induction hardening is performed at aheating temperature of 800-1000° C., preferably at 800-950° C. Theinduction hardened member thus manufactured has the prior austenitegrain size of 12 μm or less through the thickness of the hardened layerformed on the surface of the steel product and exhibits high torsionalfatigue strength.

When the heating temperature during the induction hardening is less than800° C., the formation of austenite is insufficient and therefore ahardened layer is not completely formed. This results in low torsionalfatigue strength. On the other hand, the heating temperature over 1000°C. promotes the grain growth of austenite. This increases the austenitegrain size, leading to the formation of coarse grains in the hardenedlayer and therefore to lower torsional fatigue strength.

FIG. 2 shows a relationship between heating temperature of inductionhardening and prior austenite grain size of hardened layer in aMo-bearing steel (Mo: 0.05-0.6 mass %) according to the presentinvention and a comparative Mo-free steel.

Lower heating temperature of the induction hardening in both theMo-bearing steel and the Mo-free steel gives a smaller prior austenitegrain size of the hardened layer. In the Mo-bearing steel, the heatingtemperature of 1000° C. or less, preferably 950° C. or less makes itpossible to remarkably decrease the prior austenite grain size of thehardened layers.

The grain size of the hardened layer is further decreased by repetitiveinduction hardening. In the repetitive induction hardening, not only theheating temperature of the final induction-hardening, but also the otherheating temperatures are preferably 800-1000° C.

Furthermore, in the repetitive induction hardening, the thickness of thehardened layer after the final induction hardening is preferablyequivalent to or thicker than that of the hardened layer after thenon-final induction hardening. This is because the grain size of thehardened layer is most affected by the final induction hardening; if thethickness of the hardened layer after the final induction hardening issmaller than that of the previously hardened layer, the final grain sizethrough the thickness of the hardened layer becomes larger, decreasingthe torsional fatigue strength.

The heating time of induction hardening is preferably less than 5seconds, and more preferably less than 3 seconds to inhibit the graingrowth of austenite and decrease the grain size of the hardened layersignificantly.

Low heating rate and low cooling rate during induction hardening mayenhance the grain growth of austenite, increase the grain size of thehardened layer, and decrease the torsional fatigue strength. Thus, theheating rate and the cooling rate are preferably 200° C./s or more, andmore preferably 500° C./s or more.

EXAMPLE

Steels A-Y, A1, and B1, which have compositions shown in Table 1, weremelted in a converter and cast continuously into blooms having across-section of 300×400 mm. These blooms were rolled into 150 mm squarebillets through a breakdown process. Then, the square billets wererolled into 24-60 mmφ steel bars and cooled at the cooling rates shownin Table 2. The finishing temperature in the rolling into steel bar wasover 900° C., which was suitable for the formation of a structure ofbainite or martensite.

Torsional fatigue test samples that had a parallel portion of 20 mmφ anda notch having a stress concentration factor α=1.5 were prepared fromthe rolled steel bars. These torsional fatigue test samples were heatedat a heating rate of 800° C./s to different temperatures for differenttime periods, as shown in Tables 2-1, 2-2, and 2-3, and then quenched ata cooling rate of 1000° C./s, using an induction hardening apparatusoperating at a frequency of 15 kHz. The induction hardening treatmentlike this was conducted twice or three times on some torsional fatiguetest samples, as shown in Tables 2-1, 2-2, and 2-3. Then, the testsamples were tempered in a furnace at 170° C. for 30 minutes. Theinduction hardened samples thus prepared No. 1-55 were subjected to thetorsional fatigue test under the following conditions.

The torsional fatigue test was carried out with a torsional fatigue testmachine having a maximum torque of 4900 N·m (=500 kgf·m) under differentalternating stress conditions. The fatigue strength was measured as astrength at which the sample fractured at 1×10⁵ times of torsionalcycle.

For the samples prepared under the same conditions, structure beforeinduction hardening, thickness and grain size (average prior austenitegrain size) of the hardened layer formed after induction hardening weredetermined by means of a optical microscope.

The thickness and the grain size of the hardened layer were measured bythe method described above. The grain sizes shown in Tables 2-1, 2-2,and 2-3 are the maximum grain size of the average austenite grain sizesmeasured at one-fifth, a half, and four-fifths of the thickness of thehardened layer. In the samples subjected to two or three times inductionhardening, thickness of the hardened layer after each inductionhardening, and grain size of the hardened layer after the finalinduction hardening were measured.

Tables 2-1, 2-2, and 2-3 show the results.

Samples 1-10, 12-23, and 37-52, which were prepared from the steel barshaving the compositions and the structures according to the presentinvention and were subjected to induction hardening according to thepresent invention, have a grain size of 12 μm or less of the hardenedlayer and therefore have a torsional fatigue strength of 700 MPa ormore.

The comparison between samples 1 and 2 or between samples 4 and 5 showsthat increasing the time of induction hardening from one to twodecreases the grain size of the hardened layer and thus increases thetorsional fatigue strength.

The comparison between samples 8, 37, and 38 shows that increasing thetime of induction hardening from one to two makes the torsional fatiguestrength of sample 37, which has a thinner thickness after the secondinduction hardening than a thickness after the first inductionhardening, lower than that of sample 8, which is subjected to singleinduction hardening. On the other hand, sample 38, which has a thickerthickness after the second induction hardening than a thickness afterthe first induction hardening, has a significantly higher torsionalfatigue strength than sample 8. In sample 38, the prior austenite grainsize at a depth of four-fifths of the thickness of the hardened layer is3.5 μm, and that at a depth of one-fifth of the thickness of thehardened layer is 2.6 μm. Thus, the small grain size in the surfaceseems to contribute to the increased torsional fatigue strength.

In samples 39-48, the Al content is controlled within a desired range of0.05-0.10 mass %. Thus, the grain size of the hardened-layer is smalland the torsional fatigue strengths becomes high.

On the other hand, in comparative sample 11, the total volume fractionof bainite and martensite is less than 10% because of a low cooling rateafter rolling. This results in a large grain size of the hardened layerand a lower torsional fatigue strength.

Sample 24 has a small grain size of the hardened layer. However, the Ccontent higher than the scope of the present invention results in a lowgrain boundary strength and a low torsional fatigue strength.

Samples 25, 26, and 27, which have C, Si, and Mo content lower than thescope of the present invention, respectively, have a large grain size ofthe hardened layer and a low torsional fatigue strength.

Samples 28, 29, 30, and 31, which have B, Mn, S and P, and Cr contentout of the scope of the present invention, respectively, have a lowgrain boundary strength and a low torsional fatigue strength.

Sample 32, which has Ti content higher than the scope of the presentinvention, has a low torsional fatigue strength. In contrast, sample 35,which has Ti content lower than the scope of the present invention, hasa large grain size of the hardened layer and a low torsional fatiguestrength.

Sample 33, in which the heating temperature of induction hardening ishigher than the scope of the present invention, has a large grain sizeof the hardened layer. On the other hand, sample 34, in which theheating temperature of induction hardening is lower than the scope ofthe present invention, does not have a hardened layer. Both samples 33and 34 have a low torsional fatigue strength.

Sample 36 has Si content of 0.28 mass %, which is lower than the scopeof the present invention, and a prior austenite grain larger than 12 μmthrough the thickness of the hardened layer and thus has a low torsionalfatigue strength.

Samples 53, 54, and 55, which are free of Mo, as compared with samples6, 4, and 3, which contain Mo, shows that Mo decreases a grain size ofthe hardened layer significantly when the heating temperature duringhardening is below 1000° C.

While the example is described for the torsional fatigue strength, it isneedless to say that according to the present invention other fatiguecharacteristics that involve destruction and crack extension at theprior austenite grain boundary, such as bending fatigue, rollingfatigue, and roller pitting fatigue, are also excellent. TABLE 1 SteelComposition (mass%) No. C Si Mn P S Al Cr Mo Ti B N Cu Ni Co Nb V Note A0.42 0.51 0.77 0.010 0.023 0.024 0.04 0.45 0.021 0.0022 0.0044 — — — — —Inventive B 0.47 0.77 0.55 0.010 0.015 0.022 0.02 0.30 0.026 0.00140.0042 — — — — — Inventive C 0.49 0.98 0.95 0.008 0.020 0.020 0.03 0.330.032 0.0022 0.0044 — — — — — Inventive D 0.55 0.62 0.96 0.009 0.0380.029 0.03 0.15 0.017 0.0020 0.0048 — — — — — Inventive E 0.50 0.41 0.360.004 0.015 0.069 0.05 0.51 0.015 0.0018 0.0039 — — — — — Inventive F0.48 0.90 0.60 0.012 0.020 0.025 0.04 0.39 0.020 0.0011 0.0058 0.3 0.44— — — Inventive G 0.50 0.64 0.73 0.013 0.031 0.033 0.02 0.25 0.0580.0033 0.0041 — — 0.22 0.040 0.16 Inventive H 0.48 0.91 0.60 0.012 0.0200.025 0.18 0.39 0.020 0.0021 0.0043 — — — — — Inventive I 0.48 0.90 0.590.012 0.020 0.025 0.04 0.38 0.006 0.0022 0.0045 — — — — — Inventive J0.49 0.91 0.59 0.013 0.021 0.025 0.04 0.39 0.019 0.0021 0.0044 — — —0.050 — Inventive K 0.48 0.90 0.60 0.012 0.020 0.024 0.04 0.40 0.0200.0022 0.0050 — — — — 0.21 Inventive L 0.48 0.90 0.60 0.012 0.021 0.0250.04 0.38 0.020 0.0024 0.0038 0.4 — — — — Inventive M 0.49 0.89 0.610.012 0.020 0.025 0.03 0.39 0.019 0.0020 0.0041 — 1.5 — — — Inventive N0.48 0.90 0.60 0.013 0.020 0.026 0.03 0.39 0.021 0.0019 0.0040 — — 0.45— — Inventive O 0.84 0.50 1.10 0.012 0.019 0.021 0.03 0.24 0.025 0.00180.0040 — — — — — Comparative P 0.26 0.62 0.90 0.013 0.022 0.023 0.030.14 0.022 0.0026 0.0044 — — — — — Comparative Q 0.46 0.06 0.69 0.0120.023 0.031 0.01 0.32 0.018 0.0023 0.0032 — — — — — Comparative R 0.510.76 1.01 0.018 0.019 0.019 0.02 — 0.026 0.0032 0.0041 — — — — —Comparative S 0.49 0.44 1.04 0.013 0.014 0.028 0.04 0.20 0.023 0.00010.0037 — — — — — Comparative T 0.44 0.55 2.59 0.007 0.018 0.033 0.020.28 0.035 0.0001 0.0056 — — — — — Comparative U 0.47 0.34 0.89 0.0390.083 0.024 0.02 0.36 0.025 0.0023 0.0049 — — — — — Comparative V 0.480.66 0.55 0.009 0.018 0.021 0.31 0.14 0.020 0.0019 0.0045 — — — — —Comparative W 0.53 0.81 0.93 0.012 0.018 0.026 0.03 0.41 0.150 0.00240.0045 — — — — — Comparative X 0.43 0.52 0.53 0.013 0.014 0.027 0.040.20 0.004 0.0023 0.0040 — — — — — Comparative Y 0.44 0.28 0.87 0.0130.014 0.025 0.15 0.29 0.011 0.0018 0.0042 — — — — — Comparative A1 0.420.50 0.77 0.011 0.022 0.072 0.04 0.45 0.020 0.0021 0.0045 — — — — —Inventive B1 0.47 0.78 0.54 0.010 0.015 0.065 0.02 0.30 0.025 0.00150.0041 — — — — — Inventive*The underlined values are outside of the scope of the presentinvention.

TABLE 2-1 Cooling Thickness Grain Torsional rate Bainite MartensiteFerrite of hardened size of fatigue after volume volume grain InductionTimes of layer hardened strength Sample Steel rolling fraction fractionsize hardening induction (mm) layer [1 × 10⁵] No. No. (° C./s) (vol %)(vol %) (μm) condition hardening 1st 2nd 3rd (μm) (MPa) Note 1 A 0.7 810 17.9 880° C. × 2s 1 3.5 — — 4.5 818 Inventive 2 A 0.7 81 0 17.9 890°C. × 2s 2 3.6 3.6 — 2.9 828 Inventive 3 A 0.7 81 0 17.9 1090° C. ×   14.4 — — 11.1 704 Inventive 4 B 0.9 65 0 16.6 850° C. × 1s 1 3.5 — — 3.8822 Inventive 5 B 0.9 65 0 16.6 850° C. × 1s 2 3.5 3.5 — 2.6 879Inventive 6 B 0.9 65 0 16.6 970° C. × 1s 1 4.4 — — 7.8 795 Inventive 7 B12.5  9 91 Not 870° C. × 2s 1 3.9 — — 4.7 813 Inventive formed 8 C 0.688 0 14.9 830° C. × 3s 1 3.5 — — 3.2 850 Inventive 9 C 0.6 88 0 14.9820° C. × 2s 1 2.7 — — 3.1 782 Inventive 10 C 0.6 88 0 14.9 820° C. × 1s1 2.2 — — 3.0 766 Inventive 11 C  0.08  6 0 14.9 970° C. × 3s 1 4.0 — —19.5 537 Comparative 12 D 0.7 28 0 12.5 900° C. × 3s 1 3.5 — — 5.2 835Inventive 13 D 0.7 28 0 12.5 900° C. × 7s 1 4.3 — — 10.6 719 Inventive14 E 0.8 25 0 14.2 890° C. × 1s 3 3.9 3.9 3.9 2.6 870 Inventive 15 F 0.870 0 15.3 850° C. × 1s 2 3.8 3.8 — 1.6 900 Inventive 16 G 0.7 83 0 14.1940° C. × 2s 2 3.9 3.9 — 3.8 826 Inventive 17 H 0.7 63 0 16.2 950° C. ×1s 1 3.7 — — 7.9 741 Inventive 18 I 0.8 63 0 16.9 970° C. × 1s 1 4.1 — —8.0 787 Inventive 19 J 0.7 61 0 15.8 950° C. × 1s 1 4.2 — — 7.6 815Inventive 20 K 0.8 64 0 16.0 950° C. × 1s 1 3.9 — — 7.8 826 Inventive 21L 0.8 67 0 16.2 940° C. × 1s 1 3.7 — — 5.0 822 Inventive 22 M 0.6 87 014.5 960° C. × 1s 1 3.7 — — 7.6 830 Inventive 23 N 0.7 71 0 15.0 950° C.× 1s 1 4.0 — — 7.7 817 Inventive 24 O 0.9 33 0 Not 950° C. × 1s 2 4.04.0 — 4.9 625 Comparative formed 25 P 0.5  8 0 20.7 950° C. × 2s 1 3.8 —— 17.0 548 Comparative 26 Q 0.6 35 0 14.5 900° C. × 3s 1 3.9 — — 13.9590 Comparative 27 R 0.5 12 0 13.7 900° C. × 3s 2 4.1 4.1 — 13.7 583Comparative 28 S 0.7  7 0 14.6 920° C. × 2s 1 3.9 — — 11.1 586Comparative 29 T 0.7 87 0 16.0 910° C. × 1s 1 3.6 — — 4.2 675Comparative 30 U 0.6 69 0 15.1 880° C. × 2s 1 3.8 — — 4.0 655Comparative 31 V 0.7 24 0 14.4 860° C. × 2s 2 3.8 3.8 — 3.9 657Comparative 32 W 0.6 80 0 12.8 950° C. × 3s 1 4.0 — — 5.9 562Comparative 33 A 0.7 81 0 17.9 1150° C. ×   1 6.5 — — 13.6 615Comparative 7s 34 A 0.7 81 0 17.9 700° C. × 2s 1 0 — — Unmeasur- 308Comparative able 35 X 0.7  7 0 17.5 960° C. × 1s 1 4.0 — — 20.5 543Comparative 36 Y 0.7 32 0 17.6 950° C. × 2s 1 4.8 — — 15.5 575Comparative 37 C 0.6 88 0 14.9 *1 2 4.4 3.5 — 4.0 840 Inventive 38 C 0.688 0 14.9 *2 2 2.2 4.0 — 3.5 872 Inventive 39 A1 0.7 80 0 17.8 880° C. ×2s 1 3.5 — — 4.2 825 Inventive 40 A1 0.7 80 0 17.8 890° C. × 2s 2 3.63.6 — 2.7 838 Inventive 41 A1 0.7 80 0 17.8 880° C. × 0s 1 3.4 — — 3.9788 Inventive *3 42 A1 0.7 80 0 17.8 1090° C. ×    1 4.4 — — 10.9 712Inventive 6s 43 B1 0.9 64 0 16.7 850° C. × 1s 1 3.5 — — 3.5 835Inventive 44 B1 0.9 64 0 16.7 850° C. × 1s 2 3.5 3.5 — 2.4 890 Inventive45 B1 0.9 64 0 16.7 940° C. × 0s 1 3.5 — — 4.6 817 Inventive *3 46 B10.9 64 0 16.7 940° C. × 1s 1 3.5 — — 4.8 815 Inventive 47 B1 0.9 64 016.7 970° C. × 1s 1 4.4 — — 7.6 800 Inventive 48 B1 12.5   9 91 Not 870°C. × 2s 1 3.9 — — 4.5 818 Inventive formed 49 B 0.9 65 0 16.5 940° C. ×0s 1 4.3 — — 4.8 815 Inventive *3 50 B 0.9 65 0 16.5 940° C. × 1s 1 4.2— — 5.1 816 Inventive 51 A 0.7 81 0 17.9 880° C. × 0s 1 3.5 — — 4.2 821Inventive *3 52 A 0.4 60 0 18.2 880° C. × 0s 1 3.5 — — 4.4 819 Inventive*3 53 R 0.8 40 0 13.5 970° C. × 1s 1 3.5 — — 10.6 570 Comparative 54 R0.8 40 0 13.5 850° C. × 1s 1 3.6 — — 9.5 625 Comparative 55 R 0.8 40 013.5 1090° C. ×   1 3.5 — — 14.1 514 Comparative 1s*The underlined values are outside of the scope of the presentinvention.*1 First hardening: 1100° C. × 2s, Second hardening: 850° C. × 2s*2 First hardening: 820° C. × 1s, Second hardening: 850° C. × 2s*3 0s means that heating was stopped immediately after the heatingtemperature and then cooling was started.

1. A steel product for induction hardening that consists of C:0.35-0.7%, Si: 0.30-1.1%, Mn: 0.2-2.0%, Al: 0.005-0.25%, Ti: 0.005-0.1%,Mo: 0.05-0.6%, B: 0.0003-0.006%, S: 0.06% or less, P: 0.02% or less, Cr:0.2% or less, by mass, and a balance of Fe and inevitable impurities,and has a structure of bainite and/or martensite, the total volumefraction of bainite and martensite being 10% or more.
 2. The steelproduct for induction hardening according to claim 1, further comprisingat least one selected from the group consisting of Cu: 1.0% or less, Ni:3.5% or less, Co: 1.0% or less, Nb: 0.1% or less, and V: 0.5% or less,by mass.
 3. An induction hardened member made of the steel product forinduction hardening according to claim 1, wherein the prior austenitegrain size of a hardened layer formed on the surface of the steelproduct by induction hardening is 12 μm or less through the thickness ofthe hardened layer.
 4. The induction hardened member according to claim3, wherein the thickness of a hardened layer formed on the surface ofthe steel product by induction hardening is 2 mm or more.
 5. Aninduction hardened member made of the steel product for inductionhardening according to claim 2, wherein the prior austenite grain sizeof a hardened layer formed on the surface of the steel product byinduction hardening is 12 μm or less through the thickness of thehardened layer.
 6. The induction hardened member according to claim 5,wherein the thickness of a hardened layer formed on the surface of thesteel product by induction hardening is 2 mm or more.
 7. A method formanufacturing a steel product for induction hardening, comprising thesteps of: hot working a steel consisting of the composition in claim 1;and cooling the hot worked steel at a cooling rate of at least 0.2°C./s.
 8. A method for manufacturing a steel product for inductionhardening, comprising the steps of: hot working a steel consisting ofthe composition in claim 2; and cooling the hot worked steel at acooling rate of at least 0.2° C./s.
 9. A method for manufacturing aninduction hardened member comprising the step of: subjecting the steelproduct for induction hardening manufactured by the method according toclaim 7 to induction hardening at least once, wherein the heatingtemperature of the final induction hardening is 800-1000° C.
 10. Amethod for manufacturing an induction hardened member comprising thestep of: subjecting the steel product for induction hardeningmanufactured by the method according to claim 8 to induction hardeningat least once, wherein the heating temperature of the final inductionhardening is 800-1000° C.
 11. A method for manufacturing an inductionhardened member comprising the step of: subjecting the steel product forinduction hardening manufactured by the method according to claim 7 toinduction hardening at least once, wherein the heating temperature ofall the induction hardenings is 800-1000° C.
 12. A method formanufacturing an induction hardened member comprising the step ofsubjecting the steel product for induction hardening manufactured by themethod according to claim 8 to induction hardening at least once,wherein the heating temperature of all the induction hardenings is800-1000° C.
 13. The method for manufacturing an induction hardenedmember according to claim 9, wherein the heating time of thefinal-induction hardening is 5 seconds or less.
 14. The method formanufacturing an induction hardened member according to claim 10,wherein the heating time of the final induction hardening is 5 secondsor less.
 15. The method for manufacturing an induction hardened memberaccording to claim 11, wherein the heating time of all the inductionhardenings is 5 seconds or less.
 16. The method for manufacturing aninduction hardened member according to claim 12, wherein the heatingtime of all the induction hardenings is 5 seconds or less.
 17. Themethod for manufacturing an induction hardened member according to claim9, wherein the thickness of a hardened layer formed on the surface ofthe steel product by induction hardening is 2 mm or more.
 18. The methodfor manufacturing an induction hardening member according to claim 10,wherein the thickness of a hardened layer formed on the surface of thesteel product by induction hardening is 2 mm or more.
 19. The method formanufacturing an induction hardening member according to claim 11,wherein the thickness of a hardened layer formed on the surface of thesteel product by induction hardening is 2 mm or more.
 20. The method formanufacturing an induction hardening member according to claim 12,wherein the thickness of a hardened layer formed on the surface of thesteel product by induction hardening is 2 mm or more.